Bone implants are used for bone reconstructive surgery and repair where bone loss or traumatic damage has occurred. In addition to the usual function of strengthening and filling in a repaired bone area, implants can also act to transmit forces to the surrounding bone area, to preserve the vitality of the bone. As an example of the latter function, bone implants are commonly placed in tooth-extraction sites, to help preserve the underlying alveolar bone by providing for continued force on the bone during chewing.
The most proven and accepted synthetic material for bone reconstruction and repair is hydroxylapatite. This material has outstanding biocompatibility, due to its similarity to bone in composition and crystalline structure. The most important property of hydroxylapatite is its ability to bond directly to bone and act as a scaffold for ingrowth of surrounding bone tissue into the matrix of the particles. The tissue ingrowth acts to stabilize the implant on the surrounding bone and, where the implant is formed of hydroxylapatite particles, forms a tissue matrix which helps maintain the integrity of the implant.
One type of hydroxylapatite implant which has been used heretofore is a rigid implant which is intended to be machined by the surgeon or dentist to fit the implant site. The implant may be formed of a solid block of hydroxylapatite or a porous, continuous-phase hydroxylapatite block impregnated with a polymer resin to improve the strength and/or bone compatibility of the block, as disclosed, for example, in U.S. Pat. No. 4,222,128. Alternatively, solid hydroxylapatite blocks have been formed by binding hydroxylapatite particles with a permanent, rigidifying polymer material. Implants of this type suffer from a number of disadvantages. The process of determining the shape of the bone site and fashioning the implant to fit the site is time consuming and complicates the surgery. The implant rarely is ideally shaped and therefore may work loose in the site before tissue ingrowth can occur. Such implants require suturing soft tissue to hold them in place, and may require protecting the site against movement for an extended post-operative period. Also, the implant cannot be fashioned to take advantage of undercut surfaces at the bone site which otherwise might contribute to anchoring the implant. Up to 90% of such implants have been reported to work loose within the first year of implantation.
Implants formed of loose non-bonded hydroxylapatite particles are also well known in the prior art. In preparing an implant of this type, the particles are wet with saline or the patient's blood to give the particles some cohesion and make them manageable during the implant procedure. The loose mass of particle material is placed in the bone site, where it can adopt to the contours of the surrounding support tissue. After implantation, the particle mass is ingrown with hard or soft tissue which stabilizes the mass of particles, typically in a period of a few days to weeks. The implant can thus acquire a rigid contoured fit, and can also conform to undercut regions in the site to provide increased anchorage.
Despite the advantages of loose-particle implants, a number of problems have been encounted. Loose particles are generally difficult to deliver to the site in a convenient manner, and paraphernalia, such as mixing dishes, applicator funnels, suction devices, and the like, may be required. The particle mass has little cohesive strength and very often loses its shape before the mass is stabilized by tissue ingrowth. For example, particles used for rebuilding the alveolar ridge often exhibit ridge flattening and hence a loss of ridge height before the implant particles can be stabilized by tissue ingrowth. The loose cohesion of the particle mass also allows particles to migrate away from the implant site before tissue ingrowth is complete, and this can result in implant failure and exfoliation of particles into the patient's mouth in the case of oral implants.